Integrated circuits or “ICs” are the electronic components that run computers, cell phones, and other equipment. ICs are usually mounted on printed circuit boards (PCBs) housed within the equipment. The PCBs have conductive traces that electrically connect together the ICs and permit connection to other components, such as displays, keyboards, speakers, microphones, etc. The ICs are often the electronic elements that control device operation.
ICs are characterized by their operating speed, which is often indicated using their clock rate. For example, computers may be advertised as having a 500 MHz Intel Pentium III microprocessor. The “500 MHz” designation indicates the clock rate of the Pentium III microprocessor, which is one type of IC. ICs are also characterized by their density, which represents the number of devices built into an IC chip of given dimensions, or by a measure that reflects relative density, such as line width or another critical dimension. Over time, new generations of ICs have become faster in operation and denser than previous generations. The trend is expected to continue for the foreseeable future.
IC packages have pins or leads that conduct address, control or data signals, power, ground, and possibly other inputs/outputs to and from the IC. As the number of electronic devices included in the IC increases, the power needed to operate the devices may also increase. Moreover, operating voltages have decreased in part due to an effort to prevent adjacent device structures from shorting. Operating voltages have decreased, launch voltages and noise budgets have also decreased. As voltages have decreased, the number of pins has increased to provide better signal integrity for both signals and power delivery. Consequently, future ICs are expected to have more input/output pins and operate at higher speeds.
FIGS. 1A and 1B illustrate how the performance of an IC can be affected by factors external to the IC. For example, a relationship exists between the voltage supply to an IC and its clock rate. IC supply voltage (sometimes referred to generally as “Vcc” herein) is commonly indicated as a constant, e.g., 1.5 V. In fact, the actual supply voltage fluctuates or varies to some degree over time and with the downstream load driven by the supply voltage. For example, load fluctuations may occur as the result of switching circuitry within the IC. FIGS. 1A and 1B are graphs that plot maximum clock rate or frequency for a representative IC over possible voltage supply Vcc values for that IC. It is assumed that the IC voltage supply reaches a reliability wall at 1.65 V.
With respect to FIG. 1A, Vcc is assumed to vary over a range of ±100 mV. If the maximum voltage in the range is set at the reliability wall, then the nominal voltage Vccnom will be 1.55 V and the voltage supply will fluctuate between 1.45 V and 1.65 V. Because the minimum voltage is 1.45 V, the maximum clock rate in this example is 666 MHz.
FIG. 1B shows that the maximum clock rate can be increased, yielding a faster device, simply by stabilizing the voltage supply. As shown in FIG. 1B, the supply voltage varies within a ±50 mV range. In this case, the nominal voltage supply Vccnom can be set to 1.60 V, with the voltage supply varying from 1.55 V to 1.65 V. The voltage supply minimum of 1.55 V corresponds to a maximum clock rate of 712 MHz, an increase of 46 MHz over the example of FIG. 1A, achieved simply by stabilizing the voltage supply. Moreover, even higher clock rates can be achieved if the voltage supply is better stabilized.
The voltage supply can be stabilized using decoupling capacitors. As shown in FIG. 2, conventional PCBs often include rows of individual decoupling capacitors 2 surrounding an IC, such as a microprocessor. The decoupling capacitors 2 connect in the wiring lines or traces to the IC. The decoupling capacitors are useful from an electrical standpoint, but are far from ideal. For example, the individual capacitors take up too much space on the surface of the PCB, space that could be used by other components or that could be eliminated to achieve a reduction in size. In addition, the placement of the capacitors slows manufacture, leading to reduced manufacturing throughput and higher prices. Furthermore, the distance between the decoupling capacitors and the IC is not short enough. Reducing this distance would improve electrical performance. Moreover, as faster, more dense, lower voltage, higher pin count ICs are developed, these problems will become worse. More space will be required for the capacitors, larger capacitors will be required, and manufacturing will become more expensive.